Dormant Meristems Research Articles

Meristem dormancy in Marchantia polymorpha is regulated by a liverwort-specific miRNA and a clade III SPL gene

The shape of modular organisms depends on the branching architecture, which in plants is determined by the fates of generative centers called meristems. The branches of the liverwort Marchantia polymorpha are derived from two adjacent meristems that develop at thallus apices. These meristems may be active and develop branches or may be dormant and do not form branches. The relative number and position of active and dormant meristems define the overall shape and form of the thallus. We show that the clade III SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor MpSPL1 is required for meristem dormancy. The activity of MpSPL1 is regulated by the liverwort-specific Mpo-MR13 miRNA, which, in turn, is regulated by PIF-mediated signaling. An unrelated PIF-regulated miRNA, MIR156, represses a different SPL gene (belonging to clade IV) that inhibits branching during the shade avoidance response in Arabidopsis thaliana. This suggests that a conserved light signaling mechanism modulates branching architecture in liverworts and angiosperms and therefore is likely operated in the last common ancestor. However, PIF-mediated signaling represses the expression of different miRNA genes with different SPL targets during dichotomous, apical branching in liverworts and during lateral, subapical branching in angiosperms. We speculate that the mechanism that acts downstream of light and regulates meristem dormancy evolved independently in liverworts and angiosperms.

Non-photoperiodic transition of female cannabis seedlings from juvenile to adult reproductive stage.

Vegetative-to-reproductive phase transition in female cannabis seedlings occurs autonomously with the de novo development of single flowers. To ensure successful sexual reproduction, many plant species originating from seedlings undergo juvenile-to-adult transition. This phase transition precedes and enables the vegetative-to-reproductive shift in plants, upon perception of internal and/or external signals such as temperature, photoperiod, metabolite levels, and phytohormones. This study demonstrates that the juvenile seedlings of cannabis gradually shift to the adult vegetative stage, as confirmed by the formation of lobed leaves, and upregulation of the phase-transition genes. In the tested cultivar, the switch to the reproductive stage occurs with the development of a pair of single flowers in the 7th node. Histological analysis indicated that transition to the reproductive stage is accomplished by the de novo establishment of new flower meristems which are not present in a vegetative stage, or as dormant meristems at nodes 4 and 6. Moreover, there were dramatic changes in the transcriptomic profile of flowering-related genes among nodes 4, 6, and 7. Downregulation of flowering repressors and an intense increase in the transcription of phase transition-related genes occur in parallel with an increase in the transcription of flowering integrators and meristem identity genes. These results support and provide molecular evidence for previous findings that cannabis possesses an autonomous flowering mechanism and the transition to reproductive phase is controlled in this plant mainly by internal signals.

An Evaluation of Nuclei Preparation of the Dormant Axillary Bud of Grapevine for Cell Cycle Analysis by Flow Cytometry.

Whether the division of cells of a dormant meristem may be arrested, e.g., in the G1 phase, has proven to be an extremely difficult hypothesis to test. This is particularly so for woody perennial buds, where dormant and quiescent states are diffuse, and the organ may remain visibly unchanged for 6–9 months of the year. Flow cytometry (FCM) has been widely applied in plant studies to determine the genome size and endopolyploidy. In this study, we present the application of FCM to measure the cell cycle status in mature dormant buds of grapevine (Vitis vinifera cv. Cabernet Sauvignon), which represent a technically recalcitrant structure. This protocol illustrates the optimisation and validation of FCM data analysis to calculate the cell cycle status, or mitotic index, of dormant grapevine buds. We have shown how contamination with debris can be experimentally managed and give reference to the more malleable tomato leaves. We have also given a clear illustration of the primary pitfalls of data analysis to avoid artefacts or false results. Data acquisition and analysis strategies are detailed and can be readily applied to analyse FCM data from other recalcitrant plant samples.

Comparative root anatomy and root bud development after injury in two perennial herbs.

A bud bank is a pool of dormant meristems that enable plants to resprout after injury. While the bud bank on stem organs is established prior to injury as the stem grows, the bud bank on roots is considered at least partly formed as a response to disturbance events. To date, only woody species have been examined, and the establishment of reparative buds after injury without connection to the root vascular system has been confirmed; for herbs, no data are available. We tested whether root buds are formed spontaneously or induced after plant damage by studying root anatomy following plant injury in two congeneric perennial herbs. In a pot experiment with young plants of Inula britannica (root sprouter) and I. salicina (non-root sprouter), whole aboveground biomass was removed. Roots were sampled five times at 1-week intervals after disturbance events to evaluate bud occurrence and size, root and vessel diameters, sclerenchyma areas and carbohydrate storage. Compared to non-root-sprouting I. salicina, root-sprouting I. britannica presented more secondary thickening that was connected to adventitious bud formation and improved the root storage and transport capacity necessary for resprouting. Plant injury, in contrast to expectations, did not cause increased bud formation in I. britannica, and all buds were connected to the root vascular system. No root buds were observed in I. salicina. Our study implies that plants using bud banks on roots might depend on preformed buds. Comparative studies examining more species are needed to assess the generality of our findings.

Synergistic effect of cytokinin and gibberellins stimulates release of dormancy in tea (Camellia sinensis (L.) O. Kuntze) bud.

Reactivation of dormant meristem in banjhi (dormant) shoots is important to enhance the quality and quantity of tea production. The field grown tea bushes were subjected to treatment with dormancy breaking agents such as potassium nitrate (KNO3), thiourea, sodium nitro prusside (SNP), the phytohormones kinetin (Kn) and gibberellins (GA). The efficacy of Kn and GA were comparatively lesser than KNO3 while the combination of Kn and GA (50 and100 ppm respectively) resulted in better dormancy reduction in tea buds. This observation was supported by our results from gene expression study where accumulation patterns of mRNAs corresponding to histones (H2A, H2B, H3 and H4), cyclins (B2, D1 and D3), cyclin-dependent kinase (CDKA), ubiquitination enzymes (FUS, EXT CE2), cyclophilin, E2F, and tubulin were analyzed during growth-dormancy cycles in tea apical buds under the influence of Kn, GA and their combinations. The level of these mRNAs was low in dormant buds, which was significantly increased by foliar application of GA and Kn combination. The present study indicated that the foliar application of GA in combination with Kn will help to improve quality and quantity of tea production by breaking dormancy and stimulating the bud growth.

Dormancy by communication shutdown

Plant Science Trees become dormant in winter, with encapsulated buds protected against harsh conditions. Tylewicz et al. found that, as the days get shorter, communication channels between cells in aspen trees shut down. The blocked plasmodesmata sequester the dormant meristems from growth signals.

Photoperiodic control of seasonal growth is mediated by ABA acting on cell-cell communication.

In temperate and boreal ecosystems, seasonal cycles of growth and dormancy allow perennial plants to adapt to winter conditions. We show, in hybrid aspen trees, that photoperiodic regulation of dormancy is mechanistically distinct from autumnal growth cessation. Dormancy sets in when symplastic intercellular communication through plasmodesmata is blocked by a process dependent on the phytohormone abscisic acid. The communication blockage prevents growth-promoting signals from accessing the meristem. Thus, precocious growth is disallowed during dormancy. The dormant period, which supports robust survival of the aspen tree in winter, is due to loss of access to growth-promoting signals.

Growing competitive or tolerant? Significance of apical dominance in the overcompensating herb Gentianella campestris.

As a compensatory response to herbivory, plants may branch vigorously when the growth of dormant meristems is triggered by shoot damage. Undamaged plants, on the other hand, often restrain branching, and this limitation on growth can be considered a cost of tolerance to herbivory. Restrained branching is caused by apical dominance and may, alternatively, be associated with fitness benefits in competitive environments that favor fast vertical growth. To test these hypotheses regarding selection for restrained branching, we compared the performance of two subspecies of the biennial grassland herb Gentianella campestris; the tall, apically dominant ssp. campestris and the short, multi-stemmed ssp. islandica, which shows reduced apical dominance. For both subspecies, we manipulated the height of surrounding vegetation (competition) and damage intensity in grasslands of differing productivity (high, medium, low), and examined population growth rates using matrix population models combined with life table response experiments. In the absence of damage, ssp. campestris exhibited a higher population growth rate than ssp. islandica in the tallest vegetation, however with the growth rate still being below one. In the medium and low productivity environments where the vegetation was shorter, the population growth rate of ssp. islandica was considerably higher than that of ssp. campestris as long as no more than about 50% of the plants were damaged. When plants were damaged, the apically dominant ssp. campestris showed a positive population growth rate (λ>1) and often overcompensatory seed production in all productivity levels, while ssp. islandica showed no compensation and therefore the population was predicted to decline (λ<1). We conclude that restrained branching in Gentianella cannot be selected for by competition alone, but that episodes of apical damage are required to maintain the trait. Furthermore, because of the costs of restrained branching, apical dominance should be selected against in grasslands where competition and disturbance are low.

Can trait patterns along gradients predict plant community responses to climate change?

Plant functional traits vary consistently along climate gradients and are therefore potential predictors of plant community response to climate change. We test this space-for-time assumption by combining a spatial gradient study with whole-community turf transplantation along temperature and precipitation gradients in a network of 12 grassland sites in Southern Norway. Using data on eight traits for 169 species and annual vegetation censuses of 235 turfs over 5yr, we quantify trait-based responses to climate change by comparing observed community dynamics in transplanted turfs to field-parameterized null model simulations. Three traits related to species architecture (maximum height, number of dormant meristems, and ramet-ramet connection persistence) varied consistently along spatial temperature gradients and also correlated to changes in species abundances in turfs transplanted to warmer climates. Two traits associated with resource acquisition strategy (SLA, leaf area) increased along spatial temperature gradients but did not correlate to changes in species abundances following warming. No traits correlated consistently with precipitation. Our study supports the hypothesis that spatial associations between plant traits and broad-scale climate variables can be predictive of community response to climate change, but it also suggests that not all traits with clear patterns along climate gradients will necessarily influence community response to an equal degree.

The tolerance of grain amaranth (Amaranthus cruentus L.) to defoliation during vegetative growth is compromised during flowering

The biochemical processes underlying variations of tolerance are often accompanied by source–sink transitions affecting carbon (C) metabolism. We investigated the tolerance of Amaranthus cruentus L. to total mechanical defoliation through development and in different growing seasons. Defoliated A. cruentus recovered ∼80% of their above-ground biomass and ∼100% of grain yield compared to intact plants if defoliation occurred early during ontogeny, but could not compensate when defoliation occurred during flowering. Tolerance index was higher in the summer season (−0.3) than in the winter season (−0.7). Overall, defoliation tolerance was closely related to phosphoenolpyruvate carboxylase (PEPC) activity in leaves and the subsequent accumulation of starch (∼500μmol/gDW) and sucrose (∼140μmol/gDW) in stems and roots. Thus, A. cruentus accumulated sufficient C in roots and stem to allow branching and shoot re-growth after defoliation, but it only possessed sufficient C reserves to maintain <19% seed yield in the absence of new vegetative tissue. Seed size was larger during the warm season but it was not affected by foliar damage. Seed chemical composition was altered by defoliation at flowering. We conclude that A. cruentus defoliation tolerance depends on both, the re-allocation of starch from stem and roots, and the activation of dormant meristems before flowering to generate new photosynthetic capacity to sustain seed filling.

Morphogenesis of propagules in viviparous species Bryophyllum daigremontianum and B. calycinum

The propagule development in two viviparous &lt;em&gt;Bryophyllum&lt;/em&gt; species: &lt;em&gt;B. daigremontianum&lt;/em&gt; and &lt;em&gt;B. calycinum&lt;/em&gt; (&lt;em&gt;Crassulaceae&lt;/em&gt;) has been found to proceed via embryoidogenesis (somatic embryo). In both species, all propagule organs arise from the dormant meristem derivatives, but there are morphogenetic differences at the latest developmental stages (in adventitious root initiation). In both species, the propagule genesis proceeds through the "globular", heartshaped and torpedoshaped stages. Comparative morphological and embryological analysis of propagules in the species in question and sexual embryos revealed a strong similarity in their developmental patterns not with standing their morphological variability. It has been suggested that two &lt;em&gt;Bryophyllum&lt;/em&gt; species carry the "dormancy" gene. The present study has confirmed that vegetative propagules of the flowering plants can be either embryoids or buds.

Plasticity in apical dominance and damage tolerance under variable resource availability in Medicago truncatula

The activation of dormant meristems following apical damage is an important mechanism for tolerance of herbivore damage, but its impact could vary with resource availability. Here, we examined central predictions of the limiting resource model (LRM), according to which high resource availability can support damage tolerance in plants with deterministic apical dominance, but will have limited or no effect in plants that are induced to increase branching by increased resource availability regardless of damage. We examined these predictions by studying the branching patterns of Medicago truncatula plants in response to both light and water availabilities and their effects on tolerance of apical damage. We used plants from environments that were predicted to select for different levels of apical dominance. Intact plants from the more productive and competitive population exhibited strong apical dominance and refrained from branching even under full light, whereas plants from the less productive and sparser population exhibited greater plasticity in apical dominance and readily branched under high water and light. In accordance with the LRM, these differences translated into differential responsiveness to apical damage: given abundant water, apical damage induced the activation of lateral meristems and increased pod and seed production in plants from the more productive environment, but not in plants from the less productive environment. These results suggest an adaptive association between deterministic inhibition of lateral meristems and compensatory ability, which supports the hypothesis that greater compensatory responsiveness to apical damage could be a derivative of adaptation to other environmental stresses, such as light competition.

The Effects of Dormancy Status on the Endogenous Contents and Biological Activities of Jasmonic Acid, N-(jasmonoyl)-Isoleucine, and Tuberonic Acid in Potato Tubers

The effects of storage and dormancy progression on the endogenous contents and the growth-regulating activities of jasmonic acid (JA), jasmonoyl-isoleucine (JA-Ile), and tuberonic acid (TA) were determined in potato (Solanum tuberosum L. cv. Russet Burbank) minitubers and seed tubers over several harvest/storage seasons. In apical discs (consisting of both periderm and buds) isolated from minitubers undergoing natural dormancy progression, JA content was low immediately after harvest, remained essentially constant as dormancy weakened, rose >4-fold as sprouting commenced and declined by >50% as sprouting became more vigorous. JA-Ile content was more variable; remaining constant over 7 months of storage in 1 year and increasing ca. 5-fold over the same period during a second year. The TA content of minituber apical discs exceeded that of JA or JA-Ile by >20-fold and declined steadily to ca. 50% of initial levels during storage and dormancy progression. A similar but more temporally compressed pattern was found in chemically forced minituber apical discs following a 24 h treatment with the synthetic dormancy terminating agent bromoethane. A different pattern was observed in meristems isolated from seed tubers at three stages of dormancy progression. JA content was low in dormant meristems and remained constant in meristems isolated at late dormancy and following dormancy exit. The content of JA-Ile rose gradually as meristems progressed from deep dormancy to active sprouting. The TA content of isolated meristems was >20-fold higher than either JA or JA-Ile and rose slightly during dormancy progression. Exogenous JA (0.001 to 1 mM) had no effect on sprout growth when applied to intact non-dormant minitubers but treatment with 1 mM JA-Ile inhibited sprout growth by 35%. In contrast, direct application of 10 μg JA to single-eye tissue cylinders resulted in a 29% inhibition of sprout growth after 2 weeks while JA-Ile had no effect. Treatment of dormant minitubers with either JA or JA-Ile (0.01to1 mM) had no effect on dormancy duration or subsequent sprout growth. Collectively, these data do not support a major role for JA or its metabolites JA-Ile and TA in potato tuber dormancy control but they do not exclude other roles for these hormones in tuber physiology and early sprout growth.

Reactivation of Meristem Activity and Sprout Growth in Potato Tubers Require Both Cytokinin and Gibberellin

Reactivation of dormant meristems is of central importance for plant fitness and survival. Due to their large meristem size, potato (Solanum tuberosum) tubers serve as a model system to study the underlying molecular processes. The phytohormones cytokinins (CK) and gibberellins (GA) play important roles in releasing potato tuber dormancy and promoting sprouting, but their mode of action in these processes is still obscure. Here, we established an in vitro assay using excised tuber buds to study the dormancy-releasing capacity of GA and CK and show that application of gibberellic acid (GA(3)) is sufficient to induce sprouting. In contrast, treatment with 6-benzylaminopurine induced bud break but did not support further sprout growth unless GA(3) was administered additionally. Transgenic potato plants expressing Arabidopsis (Arabidopsis thaliana) GA 20-oxidase or GA 2-oxidase to modify endogenous GA levels showed the expected phenotypical changes as well as slight effects on tuber sprouting. The isopentenyltransferase (IPT) from Agrobacterium tumefaciens and the Arabidopsis cytokinin oxidase/dehydrogenase1 (CKX) were exploited to modify the amounts of CK in transgenic potato plants. IPT expression promoted earlier sprouting in vitro. Strikingly, CKX-expressing tubers exhibited a prolonged dormancy period and did not respond to GA(3). This supports an essential role of CK in terminating tuber dormancy and indicates that GA is not sufficient to break dormancy in the absence of CK. GA(3)-treated wild-type and CKX-expressing tuber buds were subjected to a transcriptome analysis that revealed transcriptional changes in several functional groups, including cell wall metabolism, cell cycle, and auxin and ethylene signaling, denoting events associated with the reactivation of dormant meristems.

Reiteration in the short lived root-sprouting herb Rorippa palustris: does the origin of buds matter?

Regrowth of injured plants occurs from dormant meristems. Its success is known to be dependent on extrinsic factors, such as the severity of an injury and nutrient availability, although little is known about the effect of intrinsic factors. We conducted a growth-chamber experiment with the short-lived root-sprouting herb Rorippa palustris (L.) Besser, where we evaluated the role of bud origin on plant regrowth after disturbance. In particular, we investigated whether shoots sprouting from either axillary or adventitious buds differ in compensatory growth and architecture. We triggered resprouting from axillary versus adventitious buds by two levels of disturbance: (i) removal of the whole aboveground biomass, with axillary buds on the stem base and adventitious buds on roots left intact; and (ii) removal of whole aboveground biomass including axillary buds on the stem base, and only adventitious buds left intact. Despite the fact that the adventitious buds were larger and contained more leaf primordia than axillary buds at the time of disturbance, the latter preferentially resprouted when both types of buds were present. Both axillary and adventitious shoots compensated for lost aboveground biomass; nevertheless, only axillary shoots compensated for fruit production. Shoot reiteration after injury was only partial in both types of shoot; however, adventitious shoots in differed more in their architectural characteristics than axillary shoots did. We confirmed that the output of plant regeneration after disturbance depends also on intrinsic factors such as the origin of the buds.

Deoxyuridine triphosphatase expression defines the transition from dormant to sprouting potato tuber buds

Identification of molecular markers defining the end of tuber dormancy prior to visible sprouting is of agronomic interest for potato growers and the potato processing industry. In potato tubers, breakage of dormancy is associated with the reactivation of meristem function. In dormant meristems, cells are arrested in the G1/G0 phase of the cell cycle and re-entry into the G1 phase followed by DNA replication during the S phase enables bud outgrowth. Deoxyuridine triphosphatase (dUTPase) is essential for DNA replication and was therefore tested as a potential marker for meristem reactivation in tuber buds. The corresponding cDNA clone was isolated from potato by PCR. The deduced amino acid sequence showed 94% similarity to the tomato homologue. By employing different potato cultivars, a positive correlation between dUTPase expression and onset of tuber sprouting could be confirmed. Moreover, gene expression analysis of tuber buds during storage time revealed an up-regulation of the dUTPase 1 week before visible sprouting occurred. Further analysis using an in vitro sprout assay supported the assumption that dUTPase is a good molecular marker to define the transition from dormant to active potato tuber meristems.

Poplar carbohydrate-active enzymes. Gene identification and expression analyses.

Over 1,600 genes encoding carbohydrate-active enzymes (CAZymes) in the Populus trichocarpa (Torr. & Gray) genome were identified based on sequence homology, annotated, and grouped into families of glycosyltransferases, glycoside hydrolases, carbohydrate esterases, polysaccharide lyases, and expansins. Poplar (Populus spp.) had approximately 1.6 times more CAZyme genes than Arabidopsis (Arabidopsis thaliana). Whereas most families were proportionally increased, xylan and pectin-related families were underrepresented and the GT1 family of secondary metabolite-glycosylating enzymes was overrepresented in poplar. CAZyme gene expression in poplar was analyzed using a collection of 100,000 expressed sequence tags from 17 different tissues and compared to microarray data for poplar and Arabidopsis. Expression of genes involved in pectin and hemicellulose metabolism was detected in all tissues, indicating a constant maintenance of transcripts encoding enzymes remodeling the cell wall matrix. The most abundant transcripts encoded sucrose synthases that were specifically expressed in wood-forming tissues along with cellulose synthase and homologs of KORRIGAN and ELP1. Woody tissues were the richest source of various other CAZyme transcripts, demonstrating the importance of this group of enzymes for xylogenesis. In contrast, there was little expression of genes related to starch metabolism during wood formation, consistent with the preferential flux of carbon to cell wall biosynthesis. Seasonally dormant meristems of poplar showed a high prevalence of transcripts related to starch metabolism and surprisingly retained transcripts of some cell wall synthesis enzymes. The data showed profound changes in CAZyme transcriptomes in different poplar tissues and pointed to some key differences in CAZyme genes and their regulation between herbaceous and woody plants.

Rules of crown development in the clonal shrub Vaccinium hirtum in a low-light understory: a quantitative analysis of architecture

The rules of crown development in the clonal shrub Vaccinium hirtum Thunb. in a low-light understory were identified by architectural analysis, and the structure and dynamics of current-year shoots were quantified. Development started from an initial orthotropic axis, which forked into plagiotropic axes; consequently, arched stems were formed. Subsequently, a new orthotropic shoot arose from the dormant meristem on the stem. The process from orthotrophy to plagiotrophy was then repeated. Ramets developed vertically as a result of the repeated formation of such orthotropic shoots and reached a maximum height of about 2 m. These processes were mainly characterized by the sequential change from orthotrophy to plagiotrophy in stem orientation and by a sequential reduction in shoot growth, in which a long shoot forked into shorter shoots with increasing allocation to leaves relative to stems. These rules may be adaptive for efficient light capture under conditions of low light availability, in terms of a low degree of self-shading and low cost of supporting tissue. Simultaneously, these rules worked to restrict the developmental potential of crowns, including the lateral expansion of crowns and the longevity of branch systems. This may be associated with a shrub-specific, throwaway design for stems that are expendable in high-stress environments.Key words: crown architecture, branching, leaf display, architectural unit, reiteration.

Regrowth response of young black spruce (Picea mariana) trees to meristem removal and resource addition

Regrowth of an injured plant may exceed the growth of a noninjured plant (i.e., overcompensation) when (i) dormant meristems are available to replace injured meristems, (ii) the injury level is sufficient to release dormant meristems from apical dominance, and (iii) the supply of mineral nutrient and light is sufficient to support the outgrowth of previously dormant meristems. A field experiment was conducted with 5-year-old black spruce trees to determine whether these three conditions were met when 25%, 50%, or 75% of a tree's uppermost buds were removed. Subsequent shoot growth on treated and nontreated trees (0% meristem removal) was compared for each of two levels of resource supply (natural and enhanced). Enhanced levels of light and mineral nutrients were created by removing neighbouring plants within a 2-m radius of a target tree and adding 10 grams of N–P–K fertilizer per tree. After 5 and 17 months, shoot growth did not differ significantly among trees in the four meristem removal treatments (condition ii above), regardless of resource supply (condition iii above). The absence of overcompensation most likely reflects the relatively small supply of dormant meristems on treated trees (condition i above).Key words: apical meristem, black spruce, compensatory growth, injury.

Suppression subtractive hybridization to enrich low-abundance and submergence-enhanced transcripts of adventitious root primordia of Sesbania rostrata

Upon waterlogging, the tropical legume Sesbania rostrata activates dormant meristems of stem-located root primordia, leading to adventitious root outgrowth. By suppression subtractive hybridization (SSH), cDNA clones were collected that were enriched for low-abundant and water-induced transcripts. From 192 sequenced SSH clones, ranging from 64 to 1064 bp in size, 66 unigenes were obtained. Complex cDNA probes were generated to screen cDNA macro-arrays. Sequence homologies of putative up-regulated unigenes and of non-induced unigenes will be briefly discussed.